1. Field of the Invention
The invention relates generally to corrosion testing. More particularly, the invention relates to corrosion testing welded connections. Still more particularly, the present invention relates to testing welded steel joints for resistance to sulfide stress cracking.
2. Background of the Technology
Sulfide stress cracking (SSC) is a form of corrosive hydrogen embrittlement that can lead to weakening, fracturing, and cracking of susceptible metal alloys such as steel. This condition is called SSC because it requires the combination of both stress and hydrogen sulfide acting together on the susceptible metal alloy. Specifically, the metal alloy reacts with hydrogen sulfide (H2S) to form metal sulfides and atomic hydrogen as corrosion byproducts. The atomic hydrogen product combines to form hydrogen gas (H2) at the surface of the metal or diffuses into the metal matrix.
SSC has particular importance in the gas and oil industry since the materials being processed (e.g., natural gas and crude oil) often contain considerable amount of hydrogen sulfide. Specifically, exposure to hydrogen sulfide and associated SSC can cause catastrophic failure in otherwise high integrity steel.
To mitigate this problem, standardized testing procedures were developed by the National Association of Corrosion Engineers (NACE) and others. For instance, equipment that comes in contact with hydrogen sulfide gas can be rated for sour service with adherence to NACE MR0175 and NACE TM0177 for oil and gas production environments or NACE MR0103 for oil and gas refining environments. These standardized tests provide assurance that a given steel grade (and accompanying processing parameters) would be safe for use in hydrogen sulfide rich environments up to a particular stress level. A typical test includes subjecting a test sample or specimen to a high tensile load in a liquid saturated with hydrogen sulfide gas for 30 days. In general, a test sample is considered to pass the test if the sample survived the 30 day test without fracturing or showing visible cracking.
In the oil and gas industry, many types of steel tubulars designed for subsurface use (e.g., drill pipe) are welded together with friction-type welds. The area immediately surrounding each weld (approximately 0.50-0.75 inches laterally to either side of the weld) is now being required by some drillers to be demonstrated to be safe from SSC in service. Consequently, the integrity of weld areas of tubulars subjected hydrogen sulfide gas is now of principle concern in the oil and gas industry. Conventional test procedures and standards outlined by NACE do not adequately address or cover such friction-type welds. For example, NACE document TM0177 is the authoritative guideline providing specifications for SSC testing methods, and outlines specifications for several types of test fixtures as well as other parameters for carrying out SSC testing of steel. However, NACE document TM0177 does not specifically address SSC testing of welds. Further, NACE document MR0175 is the authoritative guideline for the use of various steel alloys and fillet welds in sour environments (i.e., hydrogen sulfide rich environments), but addresses only fillet-type and butt welds. Fillet and butt welds are sufficiently different from friction-type welds that the guidelines in NACE document MR0175 are generally not extended to friction-type welds.
Accordingly, there remains a need in the art for apparatus and methods for testing the durability of friction welds between steel components subjected to stress in hydrogen sulfide rich environments. Such testing apparatus and methods would be particularly well-received if they were relatively easy to implement, repeatable and reuseable, and accurately reflected conditions encountered in field.